I had this same problem myself on an old 711 valve. I stripped the threads on a brand new solenoid. It was definitely too long. I thought I just took the retainer off a new solenoid but maybe I installed the old style solenoid. They have redesigned it where the plunger is much bigger.

Legacy Irritrol Solenoids

I finally bought a kit and replaced the bonnet, diaphragm and solenoid. I also changed out my fountain head to one that restricts more than the last one. So far it's been working for 2 days. Looks Like I had 2 issues pressure and bad/legacy solenoid.

Legacy Irritrol Solenoids

All,
After letting my system bake for almost 2 weeks. I found that I'm still getting air into my system. When I bleed the air out the system works fine. Looks like I will need to add some sort of air release.